1. Field of the Invention
The present invention is generally related to aldimine oxazolidine compounds and methods of using the compounds as reactive diluents. In particular, the present invention relates to methods of reducing volatile organic content (VOC) in a preparation and improving rheological properties of the preparation.
2. Background of the Art
Polyurethane coatings are extensively used as commercial and industrial protective and/or decorative coatings. Polyurethane coatings, known in the industry as one of the toughest coatings available, are routinely applied as protective coatings on exterior walls of buildings, industrial machinery, military equipment and vehicles, commercial and passenger vehicles, and any other surface requiring a protective coating. Polyurethane systems are also used extensively as sealants and adhesives.
Polyurethane coating systems generally include an isocyanate component (i.e., polyisocyanates or prepolymers) and multifunctional monomers, oligomers and polymers. These systems also include pigments, organic solvents, and a variety of adjuvant components, e.g., surface active agents, dispersants, diluents, and fillers. Restrictions on solvent content in the atmosphere have increased efforts to provide coatings which do not contain any volatile components, but instead include components which form either the whole or a part of the hardened film itself.
Reactive diluents are used to lower the volatile content of the coatings by reducing the loss of organic solvents into the atmosphere. For purposes of the present invention, reactive diluents are compounds of low volatility which reduce the viscosity and VOC of a coating, adhesive, elastomer, epoxy or sealant formulation and become a permanent part of the formulation through chemical reaction. Accordingly, reactive diluents are added to coatings not only as a diluent, but to reduce viscosity, copolymerize with the oligomers and polymers to form a part of the coating, and contribute to the final properties of the cured film.
Some reactive diluents pose problems which limit their use in coatings. The diluents are known to produce brittle films with severe shrinkage and poor adhesion to substrates, exhibit slow cure response, poor solvency effects or color instability, ineffectively reduce viscosity, or cause serious skin irritancy problems.
Oxazolidine compounds have been used as curing and crosslinking reagents, reacting with polyfunctional isocyanates in the presence of polyols or water to form polymeric coatings. U.S. Pat. No. 4,101,527 discloses an equimolar reaction of an oxazolidine with a polyfunctional isocyanate in the presence of water to form a polyurethane coating. U.S. Pat. No. 3,941,753 describes pre-polymers for coating formation prepared from the reaction of a ketiminoalkanol with an isocyanate. Monocyclic and bicyclic oxazolidine compounds are also used as moisture scavengers in formulating polyurethane coatings as disclosed in U.S. Pat. No. 5,223,174 and U.S. patent application Ser. No. 07/624,062.
U.S. Pat. No. 4,504,647 describes aldimine oxazolidine compounds as curing agents which exhibit a long shelf life when mixed with isocyanate. However, the mixture cures very quickly into an elastic or hard polymer in the presence of water or humidity. Coatings require an adequate potlife in order to be used commercially as automotive refinish coatings and the like. Coatings such as those described in U.S. Pat. No. 4,504,647 rapidly react with water to form a gel. These coatings have a short potlife which can prevent use of the coating formulation in typical commercial applications.
There is a need for a reactive diluent which exhibits good cure response, color stability, an adequate potlife, low intrinsic viscosity, low film shrinkage, and excellent solvating or viscosity reducing properties.